text:
Is there any regulation that authorizes autopilot use when cleared for a visual approach?
Is there any FAA reference that recommends or specifically allows for the use of a coupled approach (such as autoland) when the pilot is specifically cleared for a visual approach?

I doubt there is any reference that specifically says you are allowed to use those aids when cleared for a visual approach. Typically regulatory systems do not deal in options, but in what you must or must not do. In the case of visual approaches, the pilot must be able to keep the airport in sight and remain clear of clouds. She/He is free to use or not use whatever aids might be available. Personally, I would not forego the pleasure of flying the airplane myself, although I might use the ILS to monitor my performance.

@fooot I don't think it's a dupe of that question, as it doesn't seem to address the autoland issue (except in a deleted answer.)

The general answer is no. A visual approach isn't considered an instrument approach according to ATC so there's no restriction - or direction, if you prefer - on how to fly one:

A visual approach is an ATC authorization for an aircraft on an IFR
  flight plan to proceed visually to the airport of intended landing; it
  is not an instrument approach procedure. Also, there is no missed
  approach segment. An aircraft unable to complete a visual approach
  must be handled as any go-around and appropriate separation must be
  provided.

And from the AIM 5-5-11:

The pilot must, at all times, have either the airport or the preceding
  aircraft in sight. After being cleared for a visual approach, proceed
  to the airport in a normal manner or follow the preceding aircraft.
  Remain clear of clouds while conducting a visual approach.

That means that when you're cleared for a visual approach it's entirely up to you how you approach and land, providing that you follow the rules mentioned in the AIM and  - in the case of airline operations - whatever procedures are detailed in your operations specification.
But if you're asking about autoland specifically (not general autopilot use) then the FAA does have a few relevant comments in their guidelines for CAT III operations:

Autoland or HUD to touchdown operations are required for all CAT III
  operations, and many operators use autoland or HUD for CAT II, CAT I,
  and visual flight rules (VFR) operations as well. Part 121, §
  121.579(c); part 125, § 125.329(d); and part 135, § 135.93(d) prohibit the use of autoland or HUD to touchdown in any operation unless the
  operator is specifically authorized via OpSpecs.

And:

Practice autoland or HUD to touchdown operations may be conducted at
  CAT I runways in CAT I or better weather conditions where the facility
  classification is unknown, because the flightcrew is monitoring system
  performance, visually verifying the position of the aircraft, and can
  determine whether to continue to a landing or execute a missed
  approach in CAT I or better weather conditions. Though it is not
  recommended, flightcrew monitoring and increased visibility also
  permits practice autoland or HUD to touchdown operations using an ILS
  classified as I/A, I/B, I/C, or I/T, provided the operation is
  performed in VFR conditions

I understand that to mean that autolanding is allowed in visual conditions (provided it's authorized via OpSpecs) for training/practice purposes, and since they mention autolanding use even under VFR I don't see why it wouldn't be allowed on a visual approach.


text:
My driver's license was suspended for not having insurance. Should I even bother to apply for a medical?
My insurance lapsed for a few days, and during that few days that I got caught.
Furthermore, it's the second suspension in my life because I got a bunch of tickets when I was younger.
I hold a sport-pilot license which would be invalidated if I was rejected for the medical.

Why would you be applying for a medical? Are you going for a full license?

I can't understand your question. How does a driver license relate to your medical?

Before voting to close be aware that license suspensions can lead to the loss of an existing medical or denial of a new one, IMO this is a valid question although it could be phrased a bit better.

If your drivers license is suspended, I'm pretty sure your sport pilot license is also invalid because you are required to have a current, valid driver's license to exercise your privileges. It would be the same if a PPL's medical were revoked.

It would help if you would [edit] your question and add some details - what exactly are you trying to do?  Determine if it's legal to fly sport pilot while your drivers license is expired?  Determine if you can upgrade to a private pilot's license with a revocation on your record?  Something else?

How many days is "a few days"? Do I even want you at the controls of something that could fall on my house?

For the certain answer, ask an AME (ask for a consultation, not an exam), but I've never heard of routine traffic tickets in the past being cause to deny a 3rd-class medical.  DUI and assorted drug/alcohol issues, certainly, but normal traffic tickets, no. Fix the suspension before flying (or going to the AME for that matter), but with everything in good shape this doesn't sound like a reason to be denied a medical.  IANAL, IANAAME, etc etc.

The suspension is already "fixed" in the sense that I've paid the reinstatement fee and all that, but the suspension is still on my record and I'll have to report it.

@GdD Yes, I may be going for a full license.

@A.I.Breveleri "A few days" was 10 days, and my income has gone up by 5x since then. I'll never have the kind of money problems that led to that situation again.

In that case I withdraw my objections. Good luck with the AME.

A sport pilot license does indeed require a valid driver's license OR a minimum third-class medical certificate. If you don't have a medical certificate, you cannot exercise sport pilot privileges until your driver's license is reinstated.

My driver's license was reinstated. The problem is that now there's a suspension on my record and I'll have to tick "yes" on the corresponding question if I apply for a medical.

You don't need a medical to continue exercising sport pilot privileges. You just need a valid driver's license.

I want to upgrade to a PPL. I will need a medical before doing so unless they remove that requirement.

You have two options:

Don't tell the FAA about the suspension. (Bad idea, IMO. They will find out, and that's worse than not telling.)

Make an appointment at the FSDO to talk to them about the issue and explain the situation and events that led to that, before you visit the medical examiner.  You'd be surprised how well communication (respectful, thoughtful) works with FAA employees. They have to follow the rules, but they will help you find the way to get that medical if they can.

Talk to the medical examiner first and ask what he thinks you should do.

For those of you paying attention, that was three options. LOL!

The objective, as outlined in the NPRM, for medical review of driver licenses is to determine if there are behavioral issues which may impact the judgment of a pilot.  Drugs, alcohol,  and other risk behaviors are screened and evaluated.  There are cases where excessive speeding tickets and reckless driving has contributed to the non-issuance or revocation of a medical certificate.
To be clear, common traffic violations, are normally of no concern.
The FSDO is not involved in the medical process, but is involved in the enforcement process, so officially, they can be of little help.  The Aeromedical Certification Branch of the FAA is the responsible branch.  Substance dependence, substance abuse and personality disorders which involve overt acts are disqualifying.

In my experience, anything you disclose to aeromedical can and will be used against you. I learned the hard way that you don't tell them what they don't need to know.

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text:
What is the "on condition" concept of aircraft maintenance?
"On-condition" appears to mean that a component should be "fit until failure" or that the maintenance should only be performed upon failure of the component.
This seems problematic.
Can someone help me to understand why some components are handled this way and not as part of a regular inspection schedule of some kind?

I'm not a maintenance guy, but you may be mixing 'inspection' and 'maintenance' together here.
An inspection helps you decide if maintenance is needed or not. For example, inspecting the tire tread before every flight, or checking the cylinder compressions every 100hrs. If the inspection shows that the condition of the item is still within acceptable limits, then you don't need any maintenance yet. This is the 'on condition' or 'condition-based' approach, at least as I understand it.
Another approach is to do planned maintenance at fixed time intervals whether it's 'necessary' or not. For example, replacing engine oil every 50hrs, or replacing the entire engine every 2000hrs. In this case it doesn't matter what the item's physical condition is, you perform the maintenance even if everything looks good.
FWIW, some people strongly prefer the 'on condition' approach, especially for piston engines (see this book, for example). But even they probably replace their oil at more or less fixed intervals.
If you want something more official, I found an FAA definition:

On Condition Item (Oc)
A primary maintenance process requiring repetitive inspection or test
  to determine the condition of units, systems or portions of structure
  to assure continued serviceability. Corrective action is taken when
  required by item condition as determined by analysis of inspection
  and/or test results.

The key point as I understand it is that you base the decision to do maintenance (or not) on the physical condition of the item.

Additionally, there may not be a discrete "inspection" task. A jet engine under an Engine Condition Monitoring program would have, for example, vibration and exhaust temperatures automatically transmitted and checked against thresholds and trends in near-real time. If a problem is detected, maintenance control knows about it in minutes.

@user71659.  Correct, the aim is for no (or at least fewer) scheduled inspections, based on fixed intervals such as engine hours, or number of flights, or engine throttle cycles. The last sentence of the answer is the key point.

"On-condition" appears to mean that a component should be "fit until
  failure" or that the maintenance should only be performed upon failure
  of the component.
This seems problematic.

Only if there isn't a second unit of equal capability which can take over. This is called redundancy and is the guiding principle in aircraft design. You will not find an airliner with fewer than two engines or two tires per landing gear leg. If one of them fails, the other is capable of maintaining the core functionality of both to bring the craft to a safe landing.
In case of single-unit designs (most GA aircraft use only one engine, after all), the safety concept is a low wing loading and minimum speed of 61 knots or less so an emergency landing can be performed easily. In systems design, the goal is to have a gradual failure ("graceful degradation") so the flight can be continued even if that means temporarily a higher pilot workload or less than ideal flight characteristics.
The philosophy of aviation certification is to prove that fatal consequences occur less than once per

10⁵ flight hours to persons directly involved with control of the aircraft (pilots, in other words),
10⁷ flight hours to persons accepting the risk of flight travel but not directly involved with control of the aircraft (passengers, in other words) and
10⁹ flight hours to innocent bystanders (occupants of the house the aircraft is crashing into, for example).

If you can prove that on-condition maintenance will keep you above those numbers, the procedure is fine with the authorities. You keep the unit running and monitor its condition in regular intervals. The intervals are chosen such that the probability of deadly consequences due to the failure of both parts of any redundant system will be lower than the numbers cited above, and in case of single-component systems that the probability of failure with deadly consequences for one airplane relative to the flying hours of the whole fleet is lower than the numbers above.
The accident tolerance has been higher in the past for some aviation organisations, but in general we have managed quite well lately to surpass the required minimum level of safety.

Read this long after it was posted but what an amazingly detailed answer!

@curious_cat … and I thought I only scratched the surface. Aircraft certification and safe operation is complex.

Of course! Do you have a good reference to consult? Especially regarding these numerical probabilities of fatal failures etc. My day job as a chemical engineer makes me interested in comparing our chemical safety standards to aviation safety.

@curious_cat: I learned those when I took part in the certification of one of the designs I had the luck to be involved in. This was the result of long discussions with the experts at DASA back in the Nineties who taught me a deep respect for the certification and maintenance rules. Since it is my desire to get to the bottom of topics, I probably cost them some nerves but consequently was one of the few people outside that department who fully understood how the sprockets of that system interact.

On Condition are those items which, maintenance depends on certain Conditions being met. In simple terms, maintenance depends on the condition of the item, which can be determined by regular inspections, hence the 'On-Condition' term. The maintenance required can be determined based on the conditions being met, for eg. If a part is being lubricated regularly, there would be less wear, and hence less replacement.
On the other hand, Condition Monitored items are those, on which we cant do any inspection to determine if it is wearing down or not. Hence we monitor them for failures, and get them replaced when failed. Thus called fit until failure items. Common eg are  avionics items.

An "On-condition" inspection is a non technical term, in other words it is not defined under definitions by the FAA in FAR 1.1 (Federal Regulations). However, FAR part 147 Appendix A Par(a)(1) defines non description "inspection" (on-condition is a inspection) as "to examine by sight and touch". By exclusion from other types of inspection such as "destructive", "magnetic resonance", etc - "On-condition" is an inspection able to be accomplished by an A&P using their "sight and touch". [jz pilot/A&P]

Maintenance tasks (inspections/checks) used to detect potential failures, and consequently to avoid a total functional failure, are called on-condition inspection in maintenance.
This is because machine are left in service on the condition that they continue to meet a desired physical condition and performance standards. At specific periods, the condition of technical judgement to determine failure of the item will not occur prior to the next scheduled inspection.

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text:
What are the advantages of the Airbus New Engine Option (NEO) program?
Airbus is planning to roll out an Airbus A320neo to replace its current A320 model, and they will offer this option on other models too.
How does adding more options for engines for potential customers act as an advantage for the Aircraft Manufacturer? Why wouldn't the manufacturer optimize its plane for one engine specifically, to give best performance and roll out the plane like that?

I removed the "airbus-neo" tag based on this meta post: http://meta.aviation.stackexchange.com/questions/1597 and this: http://meta.aviation.stackexchange.com/questions/1592

I'm creating airbus-neo tag (with wiki glossary), because:

Airbus plans to roll NEO for other planes as well
A-319, A-321 are more closely related than A-320ceo and A-320-neo

Thanx for pointing out though :D

before creating it, given those meta posts, ask if it is appropriate/desired. I expect not.

posting the question there :)

I think it's mainly a question of cost, it's a relatively cheap way to provide a better aircraft. Most of the performance improvements come from the engine, so simply bolting a new engine on an old frame is a very effective way of providing aircraft with better performance characteristics at minimal development costs.

Why give 2-3 options, when you can have better thrust/drag optimisation for one?

Why design/optimize an aircraft for two different engines (costing billions, maybe saving 2-3%) when you can use the same airframe for both engines (giving almost the same performance, but at a fraction of the development costs, and thus much much lower price per aircraft)?

^^My question is, why two different engines in the first place? Why not completely optimize it for the one with best results and be over with it, because optimizing it for two different engines would inherently mean that it would lead to a drop in performance by some margin

@VivianjeetSinghSudan: Note that there were always two options for A320 as there are for all other Airbus types. I suspect the reasons are actually political, but I don't have a reference for it.

People, in general (Americans in particular), like choice. Whether one option is clearly better than the other, or both are equally good, quality marketing demands that consumers be given a choice so they feel that it was their decision, and not something forced on them. Since Airbus' goal is to sell aircraft, if they can make the purchaser feel better about the purchase of an A320neo instead of a B738 (for example), they're likely to sell more.

How does adding more options for engines for potential customers act as an advantage for the Aircraft Manufacturer?

There are a couple of ways of looking at this

Why new options?
Why multiple options

Change
The A320 has been around a while, engines have improved a lot and newer types of engine exist now that were not available before, even though the A320 type airframe is still competetive.
According to Airbus, new choice of engines allow for

Lower fuel costs
Lower maintenance costs

Airbus is working on offering a new engine for the A320 known as the New Engine Option (NEO). The choice for new engines include the CFM International LEAP-X and the Pratt & Whitney PW1400G/PW1500G. The new engines will burn 16% less fuel: the actual fuel gain on an A320 installation will be slightly less, since 1-2% is typically lost upon installation on an existing aircraft. Airbus CEO said to be "comfortable" with the projections of 20% lower maintenance cost for the Pratt & Whitney's PW1000G family, compared with today's engines.

From Airbus
Choice
Speculating a little:
The people who buy and operate aircraft like having a choice of engine manufacturers. It helps keep the engine market competitive and it means they can, if they want, operate a range of aircraft all using engines from one maker - potentially allowing for savings in engine maintenance infrastructure.
Aircraft operators sometimes don't actually buy engines with their aircraft, they buy thrust from an engine maker and the engine maker (effectively) install their engines on the operators aircraft. The engines might remain the property of the maker.
It may also be that aircraft makers don't wish the future of their aircraft to be tied to a single supplier of engines. It makes negotiating harder and it makes their future prosperity dependent on the continued existence of one manufacturer.

+1 asset ownership costs are a significant expense for airlines and the savings to be gained from a well-structured lease or financing deal can outweigh the savings provided by an airframe/power plant combo that's been optimized to the nth degree.

How does adding more options for engines for potential customers act as an advantage for the Aircraft Manufacturer?

The primary reason for adding a new engine option is to increase performance. As technology improves, engines get more efficient, so putting new engines on a plane can substantially improve the efficiency of the plane. Both the LEAP-1A and PW1000G engines offered on the A320neo claim a 15% fuel burn reduction.
The primary reason for offering multiple engine options is to give the customer choice. Each engine has slightly different thrust, fuel burn, weight, and maintenance requirements. Some airlines may want to stick with certain engine manufacturers to simplify maintenance.

Why wouldn't the manufacturer optimize its plane for one engine specifically, to give best performance and roll out the plane like that?

Sometimes they do. The 737, since the Classic series, has only had the CFM56 engine option. The new 737 MAX will only have the CFM LEAP option. Business jets will also tend to only have one engine option. The 777X in development plans on only offering the GE9X engine.
The challenge is not really in optimizing the plane for one engine. Most of the plane doesn't care much what kind of engine is on it. Although there will be some points of compromise, the parts that are specific to one engine type can be designed separately, and installed based on which option the customer chooses. Creating designs for more engines is more work, which the manufacturer will have to weigh based on the benefit that more options will offer.

Well, let's take a look, what are the advantages of having a choice of engine?
The A320neo is set to offer engines from two different manufacturers: the CFM International Leap 1A, and the Pratt & Whitney PW1100G. These families of engines are designed to be more fuel-efficient, and what ever more "efficients" there are.

How does adding more options for engines for potential customers act as an advantage for the Aircraft Manufacturer?

Let's take a look at this: My friend lives in New York, my cousin lives in Amsterdam, I live in Toronto. What happens if I want to visit any of them, what happens if I want to see all of them? That would be some very expensive flights...
Airlines don't want a wide variety of things in their inventory either. Take a look at maintenance. Some people can work on one engine, while other people can't. Then, you might have unfamiliar technologies. There would be a lot of change, I'd need different people for each engine. That would be very costly too.
For airlines that already have a "predominant" engine in their inventory, they'd want to stick to it. They're familiar with the technology, the basic systems, and they have engineers who can work on this too. This is why many airlines stick to a sole engine manufacturer, or more importantly, a single airplane manufacturer.
Having different choices means you can serve a wider market. If I see that I can get this plane with my favourite engine, it's a double win! :)

Why wouldn't the manufacturer optimize its plane for one engine specifically, to give best performance and roll out the plane like that?

Many manufacturers already do. If you were to look at the Bombardier CSeries, the exclusive engine is the PW1100G. Engines aren't the biggest part. If an engine can do it's job, deliver fuel efficiency and so on... Then why shouldn't we have it as an option? If it's feasible to have more than one engine on a plane, then they'll go for it.


text:
What is a typical ratio of airline pilots to aircraft?
Delta has about 800 aircraft for their current operations; how many pilots does the company have? What is a typical or average number of airline pilots employed per aircraft in the fleet?

The carrier I worked for had a little over 5 crews (10 pilots) per airplane. We didn't do ops requiring more than 2 pilots though, so it may not be applicable to a carrier like delta.

It might be good to be a little more specific about the type of airline (for example, major U.S. airlines like Delta,) since the answers will probably vary depending on the type of operations a given type of airline typically performs. Otherwise, though, I do think this is a reasonable question and I'm voting against closure.

You can't really answer this question without knowing the type of network schedule: long haul schedules require different numbers of crewsets than short haul, and there is again an added complexity based on the sector lengths too.

A Google search shows United Airlines has 709 aircraft and 12505 pilots for a ratio of 17.6 : 1.
If an aircraft was operated 100% of the time, a month's activity (720 hrs) would need 7 pilots pulling 100 hr/month duties, so it's an interesting related question why they need twice this amount. Perhaps there's significant variation in the hours worked by pilots - not all want to work 100 hrs/month, or can be efficiently scheduled to do so because of their home locations etc.

I question that number, as it seems pretty high. But included in that number could be management pilots, check airmen, sim instructors. And also, every airline has slightly different work rules for various ways they can build schedules and trips, which effects the number of pilots needed.

Doesn't it take two pilots to crew an aircraft?

If the airplane operates 12 hours per day, it's doing pretty well. Long-haul jets might exceed that number for a while, but then they're down for maintenance checks & such. Depending on the length of the flight, somewhere between 2 and 4 pilots are required. Domestic is always 2. That 100 hours/month is the (old) regulatory maximum, but with 1000/year you couldn't average over about 83, and few pilots actually get close to that.

DOH! Two pilots per flight and now you have a need for 14 pilots per aircraft under 100% usage. But as you say, it's a lot less than that, but so are the available pilot hours, and the numbers make sense now.

Lines of flying generally have to average out to a monthly line value specified by the CBA. e.g. at carrier X, lines may average 80 hours and can not be built over 95 hours. For every 90 hour line there was a 70 hour line. Also factor vacations, recurrent training, etc and the numbers make sense.

Note that for an airliner you need at least 2 pilots per flight (the ratio goes closer to 14:1), the aircraft itself need maintenance (you technically cannot reach 720h/month), and you may not fly all night (you need airports open when departing/arriving as well as passengers; both are easier to find during the day)


text:
How is the D8 Double Bubble aircraft by NASA so efficient?
How does the 'double bubble' fuselage cross-section of the "Double Bubble D8" aircraft (developed by MIT for NASA) help improve overall efficiency by around 70%?

Image credit: NASA/MIT/Aurora Flight Sciences

Do you have a reference that shows that it actually does improve overall efficiency?

http://www.nasa.gov/content/variation-on-a-theme
The efficiency increase is claimed. I just wish to know if the fuselage cross section has any specific contribution to that

Please elaborate a bit on your question. Not every reader might know about this aircraft, and please include a clear/authoritative reference to the 70% in the question.

claimed by whom?

Read the second comment

@VivianjeetSinghSudan, you have to scroll through about 30 articles to find one on the D8. Got anything better?

Efficiency improvement compared to what?

The main advantage is its lower flight Mach number of 0.74. This allows it to use minimum sweep, which in turn reduces wing area, structural masses and thrust requirements. Now pick the right definition for efficiency (one that neglects speed), add the engine fuel consumption expected in 20 years, and the concept looks like a winner. If fuel prices go to \$200 or \$300 a barrel, this type of aircraft will lower the cost of flying substantially. The double-bubble fuselage will contribute only a little to the overall savings, though. Most will come from the (hypothetical!) engines and the high aspect ratio wings with little sweep.
Note that the lower flight speed is compensated by the claim of much shorter boarding times, so the double bubble comes out ahead in block times. The only reasoning is that passengers will board more quickly because they can choose between two aisles to get to their seats. Very questionable!
Below is page 105 from this report which shows the fuel consumption improvement of the separate steps from a Boeing 737 reference design to the D8.1 concept. The biggest contribution is from lowering cruise Mach (and increasing cruise lift coefficient, which allows to reduce wing area) and from engine optimization.

As it looks now, the marginal cost of fracking will keep oil prices below $70 for the next years, and airlines will keep ordering planes which look like those they operate today.
Looking through the proposals in the PDF linked in your comment, most of them make me cringe. Since the Seventies we see the same concepts for supersonic travel, and here they are again, only presented in different colors. Some subsonic designs are sound, but again we see hopeless concepts like the boxed wing which is kept alive by a stream of NASA grants, only because it looks so different. I am equally skeptical of those blended wing concepts which pop up with every new generation of aircraft designers.

It sounds like a straighter wing, lower cruise speed and more efficient engines would get similar gains from a 'traditional' cross-section 737. Am I hearing you correctly?

@FreeMan: Yes. Look at turboprops for proof.

Just to add to this, it is still more efficient even at M0.8 than a 737-800 with the same engine, and without the benefit of BLI, and that is due to the lifting body which makes it sort of a quasi-BWB. We have known for a long while that BWBs are more efficient than tube-wing structures.

@interested22 Oh, please do not believe all that nonsense. Of course they are not, or they would long be flying already.

Looking at MIT's presentation, more specifically pages 13 - 15, they're counting on the fuselage being a lifting body, among other things.
For most aircraft, the fuselage encompasses the passenger cabin, cargo cabin, fuel tanks etc. and is designed to impose as little drag as possible in the process.  With a lifting body, the fuselage actually provides lift, instead of just adding minimal drag.
The concept is also counting on

cleaner aerodynamics
advanced, lightweight materials
higher-efficiency engines
less wing area

with the fuselage providing some lift, the lighter weight and less fuel needed for the projected mission, less wing area is needed

All of these, in combination, provide the estimated 70% greater efficiency.

Due to the way induced drag works, fuselage providing lift will add more drag than if the lift generation is left up to the wings.

Their presentation on page 87 specifically covers reasons for the fuselage design.
Additional lift and more cabin room in a smaller fuselage seem to be the main reasons they give.  But there are a lot of other details in the presentation too.
But note: the fuselage shape is only part of the overall claimed efficiency improvements.  There are many other features such as a lower cruise speed, reduced sweepback of the wings leading to weight saving, etc.


text:
What pieces of evidence do investigators have access to in the MH17 crash investigation?
In September 2014, Dutch investigators published a preliminary report for the MH17 crash. I understand they based there report on radar records, CVR, FDR and photos and thus can only say the flight goes as expected until multiple impacts from unidentified projectiles came from outside the plane.
I thought it was easy to identify those projectiles based on fire traces and chemical analysis on remaining wreckage, but it seems they were not able to do such analysis.
So my question is: what evidence do they have to perform their investigation? If they have wreckage, how can these be pieces coming from the aircraft? Based on the pieces they recovered (and ignoring the political background due to the Ukrainian situation), what kind of conclusion can be expected?

In the Hollywood the residue on the impact points would take the investigators straight to a shortlist of possible sellers, unfortunately this isn't a movie.

@ratchet freak. I know, but identifying a kind of explosive is feasable with chemical analisys, thus rejecting some of the possible weapons.

That assumes enough chemical residue remained after a week in the wind and rain.

Indeed. But I don't know if those wreckages have been recovered and kept inside, and if it is the case if they are available to investigators.

"If they have wreckage, how can this is pieces coming from the aircraft?" -- what do you mean? How can they be assured these are pieces coming from the aircraft?

They examined the pieces, they did not recover them (they are still there out in the fields). Also, this is a preliminary report, not the final one. And, this is an investigation to find out the causes of the crash: extreme damage from foreign object is a suitable conclusion; finding who launched said foreign object is not part of that investigation.

Air Accident Investigation
Evidence available to the air accident investigators can include

Flight data recorder.
Cockpit voice recorder.
Aircraft wreckage.

residues, burn marks, punctures, tears and evidence of stresses.

ATC communication recordings and controller statements.
Airline records.

Aircraft maintenance records
Pilot schedules, dates of health checks, training, hours on type etc.

Radar and ADS-B data (position, speed, altitude etc).
Technical data transmitted in flight to airline, engine maker etc.
Photographs, measurements and other reports from investigators at crash site.
Eye-witness accounts from people on the ground.
Relevant NOTAMs.
Data from other aircraft in the vicinity at the time and in previous and following days (e.g. typical tracks and variations over eastern Ukraine).

In the case of MH17 the situation is obviously made more difficult by the armed conflict at the crash site and the wider political conflicts. Investigators have to take into account that some evidence may have been tampered with, destroyed or fabricated.
As others have noted, the purpose of an air-accident investigation is not to determine legal liability. Where needed, there is an entirely separate criminal investigation for that purpose.
Until they publish their report, we won't know whether the evidence available to them is sufficient for investigators to make any recommendations for changes by manufacturers, operators or regulators to reduce the number of future future occurrences of this type of incident (e.g. recommending airlines don't fly at any height over areas where there is armed conflict).
Other Investigations
Evidence relevant to other types of investigation might include

US records of alleged missile launches detected by strategic/tactical missile detection satellites.
Ukrainian alleged intercepts of separatist radio communications.
Online material allegedly published by Ukrainian separatists.
Online photos and videos alleged to be of Russian missile launchers in transit through the Ukraine-Russia border regions.

It is difficult to see how the conditions could ever be conducive to a satisfactory judicial investigation of this material acceptable to all parties with an interest in the incident.

I have the impression you are mixing two different types of investigation:

criminal investigation (interested in finding the culprit of a crime)
Aircraft Accident and Incident Investigation (whose purpose is the prevention of similar accidents and incidents cit from Page 3)

The investigation you cite is of the second kind: they are interested in finding out whether the loss of MH17 was due to some aeronautic-related cause (pilot training, maintenance, manufacture, etc.) and eventually how to avoid similar losses in the future.
In this preliminary report they state that the cause of the loss was the impact with several foreign objects. This is a prefectly accebtable conclusion for such investigation.
You were expecting instead an investigation and a report of the criminal kind. For what I know, you will have to wait quite longer for that.

I agree this is a suitable conclusion. In the purpose of explaning the causes of the accident, something more precise would be welcome. (e.g. the kind and the trajectory of the projectiles, the systems damaged by thos projectil,...) My question remains: what are the pieces of evidence investigators have? is this enough to explain the causes of the accident?

@ManuH the systems damaged by those fragments will probably be listed in the final report (if deemed useful information). The pieces of evidence used are listed in the released report. The future intermidiate reports may contain an assesment on whether additional data is required.

In the full report by the Dutch Safety Board, there is a list of all materials retrieved from the crash sites. These include:

Cockpit Voice Recorder
Flight Data Recorder
Remains of the crew and passengers
Personal belongings of the crew and passengers, including data carriers such as smartphones and cameras
Parts of the wreckage
Parts of a surface-to-air missile and its warhead

Additional information available includes:

UkSATSE secondary surveillance radar, both raw data and processed data
A video film of a radar screen showing processed secondary data from UkSATSE
ADS-B data from UkSATSE
A video film of a radar screen showing processed primary and secondary data from GKOVD

At the Gilze-Rijen Air Force Base, a reconstruction of part of the aircraft was made with the found parts of the wreckage. This was used in the investigation and the subsequent court case.

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/


text:
Why wasn't MH-17 flying at 33,000ft before it entered Ukrainian airspace?
I was looking at the path of the MH-17 flight that crashed in the Ukraine on FlightRadar24 and noticed it was flying at FL310 (31,000ft), then because of the Ukrainian NOTAM which went up to FL320 (32,000ft) and below, it then ascended to FL330.
Why wouldn't it be flying at FL330 already from the start? Doesn't a higher altitude mean less air resistance (because the air is thinner) and therefore better fuel efficiency?

This is normal.
A 777 on the AMS-KUL route cannot climb to FL330 right after takeoff - this would only be possible maybe 3-5 hours into the flight.
Airliners fly as high as the available thrust (and ATC) allows, because the thinner air allows to fly at a higher lift coefficient and less drag overall. The limit is given by the combination of available thrust and the actual mass of the aircraft, and it will climb higher during a trip as fuel is burnt off. The maximum range is possible at one particular lift coefficient, and to maintain this during a long trip means to increase altitude, so the lower density compensates the lower aircraft mass. Engine thrust is normally sized during design such that this optimum altitude can just be reached in normal operation.
On eastbound routes in controlled airspace, pilots are encouraged to fly at odd thousands of feet (depending on the local rules), so the climb will not be continuous, but stepwise. See this answer for more on separation rules.
For a rather lengthy answer with more background on the theory how to pick the ideal flight condition for maximum range, see this link.

An interesting example of this is the cruise-climb that was used by the Concorde.  As there were no other aircraft that could fly at the desired altitudes, it was given a block altitude and would gradually climb as it got lighter, allowing for an increased range.

Also, to add to Peter's answer.  Winds change with altitude.  So, if the wind happens to be a strong headwind at a higher altitude, it can be more efficient to stay lower, depending on the specifics of the airplane and the wind profile.  Note that I have no looked at the specific winds that were present during MH-17.

As Peter correctly notes, airliners which take off heavily laden with fuel and passengers struggle to reach their maximum altitude capability until they have burned off sufficient fuel. They achieve this by a technique known as a "step climb".
In this technique, they keep stepping up from one altitude to a higher one as their performance allows.
Prior to reaching Ukrainian airspace, MH17 would have flown through rather dense airspace over Europe where they may have had quite strict altitude stipulations, so that even if it had the performance to climb higher that may have been limited by other traffic inbound to Europe flying over the top of them.

In addition to the other answers, they didn't have to either.
As can be seen in this visualisation made by the Dutch Safety Board, the restrictions at that time concerned the eastern part of Ukraine; not the entire country.

(Source: Dutch Safety Board, investigation Crash MH17, 17 July 2014, appendices)
For those interested, the NOTAMs are included in the appendices.


text:
Whenever I hear a plane throttling down quickly, what is happening visually?
I am completely blind and have heard this several times when I was at air shows. Then my friend sent me this recording of a crop duster spraying the fields, and I heard the same type of sound.
It starts out by slowly ascending, and then the plane's engine rapidly decreases its speed.
I know that because of the Doppler effect, sounds can sometimes be deceiving, so I wondered what I would hear if I were inside the plane.
Here's the recording. It's audio, unfortunately, but hopefully someone with experience will be able to provide clarification.

There is no link for the audio recording I'm afraid.

I'm not sure why, but when I post an Iframe, it shows up in the edit body, but it doesn't post as an actual frame where the embedded player should be. https://clyp.it/a3mczu2m

I fixed it for you by editing your post to put in the link.

Ok I listened and what you are hearing is a turbine powered ag plane.  Most of the rising and decreasing pitch you are hearing that sounds so dramatic is doppler effect from the propeller.  The propeller speed will be constant all this time as the pilot will set the RPM for his activity and leave it, just changing power with throttle.
So in the cockpit, the noise from the prop will more constant without the rising and falling sound, just a steady OOOMMMM of somewhere around 100 hz (three blades at 2000 rpm say), not as intense as the sound heard from the ground, with a roar/hiss from the air passing that rises and falls somewhat with speed as he dives and zooms.
The intensity of the propeller sound will also increase or decrease a lot with the power, but not the overall pitch; because the propeller is constant speed, that remains constant unless the speed is changed, and that won't happen during this maneuvering.
Then the very high pitch zeeeeeeeeee whining sound under the big booming sound is the turbine engine itself. It's sound will change a little bit from within the cockpit as power is added and removed, with some change in the pitch of the whining sound and its intensity.
This video is from the inside of a plane like the one you heard, but the inside sound is being recorded through an active noise cancelling headset, so 80% of the cockpit sound is being suppressed.  But you can hear faintly the airstream, and a bit of the prop's sound and can tell that it's pretty constant to the pilot the entire time.  Way less dramatic for sure.

So is it a turboprop? I've been in one (a Cessna-208) for my sky-dive, and its propeller RPM sounded like 105-110 RPM.

@HeavenlyHarmony on Cessna 208 Caravan, the propeller spins 1600 to 1900 RPM in cruise, a bit more on take-off and landing. Generally the faster the aircraft, the slower the propeller spins, independent of the engine type. So in small airplanes up to about 2700 RPM while regional turboprops like ATR-72 go down to somewhere around 1000 RPM. But nothing goes much lower than that in flight, and the slower propellers tend to have more blades, which raises the sound pitch again.

@HeavenlyHarmony Yes in this case it's a turboprop.  The whine of the turbine is quite distinctive.  The dominant sound you are hearing in the cabin is propeller wave pulses from the blades passing impacting the windshield, so the frequency is the number of blades passing per second.   If it sounds like 110 Hz sound, and the prop has three blades, it's spinning at 2200 RPM, which is probably about right for a Pratt & Whitney PT-6 turboprop not at takeoff power.

What you are hearing is indeed the doppler effect. As the plane approaches you, the pitch rises, and as it flies away from you, the pitch falls. Inside the cockpit the pitch would stay constant.

So why, then, does it sound more dramatic than if it went normally? Usually when you hear a plane approaching, it's already at the peak of its pitch, so all it does is decrease the pitch much more slowly as it receeds into the distance. So why does it sound more dramatic in this case? Is it because the plane is going by much faster, or because it is doing some other manoeuvre?

@Heavenly The plane approaches much closer than usual. When it comes this close, it goes from coming straight at you (high pitch) to going away (low pitch) almost instantly. Normally planes are much further away and the transition from "incoming" to "outgoing" is much slower.


text:
What kind of motion will an airplane enter into when disturbed?
When an airplane is disturbed from a longitudinal equilibrium position, what sort of motion will the aircraft experience? Will it be a phugoid or a short period motion? Or will it be both?

Isn't longitudinal displacement simply the same as forward motion?

To continue @RalphJ 's comment: meaning you are asking about a disturbance in airspeed?

@RalphJ Longitudinal motion comprises of motion in X, A direction and pitching moment.

The separation into short-period and phugoid motion is somewhat artificial. There is just motion. It just happens that when you fly a 'real' airplane (or solve its equations of motions), you'll have quite different timescales for angular motion (isolated pitch; low seconds) and linear motion (changes of altitude and speed; tens of seconds and even minutes).
Broadly speaking, heavier and faster airplanes with swept wings demonstrate more of this separation. At the other end of the spectrum, smaller airplane models have similar timescales for both the short- and long-period motion, and for them the separation is not meaningful (which makes it more difficult for analysis!)
So, to answer the question, both will usually occur. In some cases, depending on the airplane and your point of interest, you can ignore one or the other. On airplanes where separation is significant and motion is stable and well damped, it may also be possible to produce a disturbance that results in (practically) one kind of motion: either short-period (e.g. a small elevator impulse) or long-period (e.g. a brief application of pitch-neutral air brakes).

Wikipedia suggests that both happen, and that short period motion should be damped out in a second or so for an aircraft to be certified, and that phugoid motion should be gentle enough that a pilot corrects it almost without thinking.
This is the result of careful design and testing, plenty of prototypes weren't that forgiving.

it means whenever an aircraft is displaced, say due to upward gust, the aircraft will undergo phugoid and short period simultaneously, right ?

The answer depends on whether the plane is designed for quick responsiveness to control inputs or hands-off stability. Hands-off stability requires the plane to exhibit critical damping in its responses to perturbations whereas quick responsiveness requires something closer to underdamped response, in which control inputs from the pilot are required to prevent oscillation or divergence.


text:
How are international departure clearances issued from an air traffic flow management perspective?
Recently, there was an incident involving a Lufthansa A350 flying to SFO which had to divert to OAK due to (at least what seems to me like) poor traffic management. In a nutshell, the approach controllers couldn't make room for an ILS landing; domestic traffic was landing using a visual approach, but Lufthansa's procedures demand ILS at night time.
This made me wonder to what extent long-term, but also instantaneous, traffic volumes are being considered when approving flight plans and (considering departure delays) issuing corresponding take-off clearances. Shouldn't somebody along the way have been aware of the IFR requirement and subsequently pushed back on either the flight plan or on issuing the departure clearance?
Even if this might have been an edge case, I'm now generally wondering to what extent international flight plans are coordinated and clearances granted "end-to-end". Do approach controllers know about all scheduled inbound flights and their instantaneous status (delays due to delayed departure, weather etc)? Can they delay intercontinental take-offs?
In Can airlines schedule more flight arrivals than the destination airport capacity?, this question is answered for domestic and intra-European flights, but I wasn't able to find any information on whether such mechanisms (CTOT, EDCT, ground delays etc.) also exist for long-haul traffic.

The "poor traffic management" in that case looks to be all & entirely within NorCal Approach, and very little that any sort of management upstream of that could have done to affect the outcome. The aircraft departed late, later than any flow control would be likely to direct (especially given good weather at the destination), so not like they could have finessed the timing to make things better. From all I read, it really looks like an arrival controller got obstructionist, and perhaps out of line... take a visual approach you're not allowed to accept, or else divert. Whoa, really???

Good point, I indeed mixed up flight rules and approaches here. I updated my question.

@RalphJ IIRC the approach controller offered the ILS approach as soon as a gap in the incoming flow of visual approaches would allow one and put the LH flight in a holding pattern. However, the LH flight did not have enough fuel to hold long enough and ended up diverting.

It is certainly not the first time that a LH flight arrives at night in SFO, so it is hardly a surprise to NorCal approach that LH cannot accept the visual. Short distance flights that popup suddenly can be an unpleasant planning surprise, but with a flight time of more than 10 hours, NorCal approach can't really claim that they didn't see this one coming either. Sub par performance by the FAA on this one.

Totally off-topic, but the first few comments on the linked article were very civil and well written. Somewhat unusual for the internet in general. A nice find!

To issue a departure delay to an aircraft you need to have reciprical arrangements with the country that it departs from. In the FAA they don't have that with all countries.
From my experience visiting the FAA Air Traffic Control System Command Center in 2018 they only had agreements with Canada and were looking at establishing with Mexico.
From my experience in Australia, we have many international arrivals into our large airports, and we have to set them up so international arrivals are considered in the sequence. We don't provide ground delays to long haul flights, there is too much variability to them and as was stated in the Lufthansa incident the aircraft departed late. This happens all the time with long haul flights.
Additionally, as the aircraft transits different airspace weather forecasts change, and level requests are not given at the time wanted due to traffic. This can both slow down and speed up an aircraft.
You have multiple airlines that will not accept a visual approach, especially the Asian and Middle Eastern carriers. When the aircraft is in the air, and you are getting regular updates on it passage through each FIR the destination system can be configured to see these aircraft coming.
In Australia we program our sequencing tools to automatically provide that spacing to that aircraft.
IMHO someone dropped the ball in the pre-planning of the sequence. Mainly the flow controller/s did not recognise the extra space required. But the justification for not vectoring 40-50 aircraft for the space for one due to safety is sound. Safety before efficiency.
This is a case of the requirements of the aircraft not being identified early enough to provide an efficient service.
To give you an example of this from Perth Australia and how we handle it Down Under. All aircraft that land internationally from the west of Perth are required to only be given a maximum of 5 minutes delay and half of them will not accept a visual approach during the day and even less at night. This is all fit into the sequence everyday with little issue, but none of them are included in the ground delay program. The ground delay program watches for their departures and updates the slots as they get closer. So, an aircraft that is late will free up restrictions earlier in the day and create more restrictions later in the day. This is then managed by the airlines as the trade slots in the program between aircraft.

Thank you, very interesting! So in other words, unlike for domestic flights, nobody explicitly tells inbound long-haul flights whether to (not) depart, and it's on the destination controllers to slot them in in real time? And regarding "seeing these aircraft coming", do controllers have visibility into the traffic flow of neighboring FIRs?

In Australia we know about all aircraft that are 90 minutes out from our airspace and then get updates every major event from them. If we have a surveillance sharing arrangement we can get accurate positions, but the procedural information is enough for us. We have no major airports within 45 minutes flight time of the FIR boundary outside of the Australian administered airspace. In the case of the Lufthansa incident that aircraft would have been known the FAA for a good hour plus before it arrived.

In Europe only flights departing from ATCFM area and some adjacent FIRs are subjected to slot allocation. The flights from airfields outside of these areas are considered as cleared and taken in account in flow planning using the estimated/revised time over the boundary of ATCFM area. Of course they will affect 'domestic' traffic but do take in account that major airports have airport slots for international flights.
Also as a matter of trivia there is an option when declaring capacity at an airport to limit the flights by minimum RVR .
For more info check CFMU manual and CFMU NOP


text:
Was radio navigation used for oceanic crossings before GPS? If not, why not?
As far as I understand, before the availability of GPS and other GNSSes, airplanes crossing large distances over water, beyond the range of ground-based navigation aids such as VORs and NDBs, largely used a combination of magnetic navigation and dead reckoning (whether manually or via an IRS).
This seems to have worked reasonably well – with the exception of some very high profile incidents such as the shootdown of Korean Air Lines 007.
Why weren't available phase comparison or time-of-arrival systems such as Omega (introduced 10 years before KAL007) or Loran-C used more widely by airliners?
Or were they, but just not uniformly and ubiquitously (at least not on that particular flight, as far as I can tell from the reports)?
Note: This question originally contained a sub-question on NDBs, which is now separate: What limits automated direction finding from working over oceans?

Note that Korean Air Lines 007 was equipped with an Inertial Navigation System, it was just used incorrectly on that flight.

True, but having external navaids available could have helped spot that incorrect usage via cross-checking, right? That's why modern FMSes use a combination of all available position sources (GNSSes, ground-based navaids, INSes), is my understanding.

Do you really want to "lump" in "manual" dead reckoning with IRS/INS?  Those are two radically different things.  Once you have a sophisticated way to precisely measure every acceleration, I'd say that you are no longer in the realm of "dead reckoning".    Sure, both concepts involve integration, but integrating precisely measured accelerations is a lot different than integrating estimated velocities.

As you acknowledge hyperbolic navigation systems were used to navigate, you should change the title of your question for something more representative of what you are asking for.

@mins: That's exactly what I'm asking in the first bullet: Were they actually (commonly) used?

@quietflyer: I realize that the two are orders of magnitude apart in quantitative accuracy, and modern INSes can be extremely accurate. What I mean is that they are qualitatively not tied to any external reference, unlike radio-based or even magnetic navigation.

For "what prevents NDB to be used" on long distances, see What is the night effect? (ionospheric reflection).

I'm aware of the night effect; I've linked your excellent answer in my question :) So does that also apply to VLF propagation?
I was initially under the impression that the frequencies used by Omega were somehow immune to it, but it seems like that system might rather have modeled it in a complex manner and corrected for it in the receivers (see also https://apps.dtic.mil/sti/tr/pdf/ADA285948.pdf).

The crew of KAL 007 believed that they were on course, because the needles were centered. Having a LORAN or OMEGA or GPS input into their navigation solution wouldn't have changed anything, because the waypoint to which they were navigating wasn't the one they should have been flying to. While we can Monday-Morning-Quarterback things all day long & decide that "if only they'd crosschecked {something} then the accident wouldn't have happened", it doesn't accomplish very much.

"does that also apply to VLF propagation", yes VLF (mostly) propagate using a tunnel between the D-layer and the ground. Of course the signal can be averaged on a long period to cancel or at least minimize the error.

I suppose this question really is two questions in one: Were radionavigation systems used for oceanic crossings before GPS (and if so, which ones), and why can't NDBs be used for long-distance triangulation. I'll split the latter one out into a different one to focus the discussion here!

It's now here: https://aviation.stackexchange.com/questions/105480/what-limits-automated-direction-finding-from-working-over-oceans

The Omega system was widely used by the airlines, but it was inaccurate, often by many miles. It main purpose was to get you close enough to land to start using more conventional radio navigation systems. I worked on them in the 1990s.


text:
Why at increasing BPR the efflux area should be enlarged (at high BPR)?
I am working on a university project. I have to comment on this chart (that represents efflux area variation with the variation of the BPR).
Every other engine data are kept unchanged (turbine entry temperature, flight altitude, flight speed, compression ratios ...)

$ BPR \le 7 $
It is clear that at low BPR the area of the nozzle should be enlarged in order to let the flow through at sonic conditions.
$ BPR \ge 7 $
What I can't explain is the reason why at high BPR the area of the nozzle useful for "swallowing" the propellant flow should be enlarged.
My explanation (is it right?)
I think that this is due to the constant (approximately) slope of velocity decrease at nozzle exit and the progressive slower decrease of the flow.

Is this the reason or there are some other explanation?


text:
Why did this aircraft do a ‘spiral’ before landing?
There was this WizzAir flight two days ago, from BUD to PMI, and right before landing it did a ‘spiral’ like maneuver. I’ve attached a screenshot.
Why did it do that? It looks strange.

At the time WZZ1161 / W62383 arrived at Palma de Mallorca (PMI), it was rush hour. To sequence the aircraft for final approach, Air Traffic Control usually give aircraft vectors (headings) to increase their path lenght so that they line up with the runway at the right time, with the appropriate amount of spacing from the preceding aircaft.
In case of peak traffic a lot of time may need to be absorbed before the final approach can start. Since vectoring causes high workload for the air traffic controller (multiple instructions, aircraft moving all over the area) ATC resorts to putting aircraft in holding patterns or instruct simple 360 degree orbits.
In the screenshot below you can see 6 aircraft on approach to PMI at the time that WZZ1161 was arriving. All had complex vectors and/or 360 degree orbits as part of their approach. There were just too many aircraft arriving for the airport to handle it efficiently.

The only published Standard Terminal Arrival Routes (STARs) over Menorca are coming from the RIXOT and MORSS waypoints as shown in the following Jeppesen chart:

It looks like your plane was coming in from RIXOT (where it should have been at FL230 or below). The only published holding is over the Capdepera VOR (CDP), but your plane was circling before reaching that point, meaning it was either instructed to circle there by ATC or requested to do so themselves.
The likely reason for circling (as discussed in the comments) is to bleed off some altitude, which raises the question why the aircraft was too high in the first place. This could have been due to traffic, i.e. ATC does not allow an early descent due to conflicts with other planes, or because the crew expected to fly the full procedure. In the chart above, you can see that the RIXOT 2M arrival goes all the way to the Mallorca VOR (MJV), which is south of the airport. This would typically be flown when approaching on runway 06L/06R.
If the wind direction just recently changed, the airport would switch to landing on 24L/24R instead. While there are published approaches to these runways from MJV, it is much more common to approach via CDP or get radar vectors to final approach. If the crew expected an approach to 06L/06R, they would suddenly be at a much too high altitude. The best way to reduce the altitude now, is to circle before accepting radar vectors on to final.

Another reason for the 'spin' is to give time for the aircraft in front to either take-off (departure) or get off (landing) the runway. Its actually called an 'orbit' as in "left hand orbit for traffic". This is if you're far out enough.. too near and you'll be going around.

The circling occurred almost half an hour before landing, I doubt this was the reason.

I still think there was an issue on the runway.. either another plane took its time getting off or maybe ground team clearing debris reported by a prior aircraft. One aircraft doing orbits may be a problem with that aircraft but two means a sequencing issue.


text:
Confused about best range speed and Carson speed
I have read that best lift /drag ratio gives maximum range which makes perfect sense to me.
I have also read Carson speed gives the best fuel consumption. which doesn’t make sense because this would mean Carson speed would give better range.
Seems to be a contradiction. Can anybody explain what this means in a way that makes sense thanks.

Consider telling us what "FF" is.  If it is "fuel flow", did you mean to say "maximum" rather then "minimum"?

For everyone whose's never heard of Carson speed: https://cafe.foundation/v2/pdf_tech/MPG.engines/AIAA.1980.1847.B.H.Carson.pdf and https://mooneyspace.com/topic/15053-carsons-speed/

Carson speed does not give the best fuel consumption (depending of course on your definition of "best"). To cover a given distance with the least amount of fuel in a piston airplane requires you to fly at the best L/D speed (indicated airspeed). Carson argues in the paper linked by @RalphJ that most piston GA aircraft are usually operated at speeds much higher than best L/D, because the best L/D speed is actually relatively slow, as those airplanes have to be designed for other requirements as well, like takeoff and landing performance.
Carson's idea was to look at how much speed you can gain from using more fuel, and the relationship between how much extra fuel you need to use to achieve a certain increase in speed is a curve that has a minium at the Carson speed. That means, between best L/D and the Carson speed you will use increasingly more fuel, and the fuel flow per distance will increase (less economic in terms of fuel cost), but the gains in airspeed (more economic in terms of time spent?) will get increasingly better, up to the point of the Carson speed. Beyond that point you will have to increase your fuel flow more and more to gain more airspeed.
To make this a little clearer and put some numbers on it, that may or may not resemble a normal light aircraft.

mass: 1000 kg
wing area: 12 m²
aspect ratio 6, efficiency 0.85, $c_D0$ 0.05
fuel mass 75 kg, energy content 40 MJ/kg, propulsion efficiency 0.3

Maximum endurance:

airspeed: 29 m/s
endurance: 6.73 h, fuel flow: 11.1 kg/h
range: 703 km, fuel flow: 10.7 kg/100 km
"fuel flow per airspeed": 0.369 (kg m)/(100 km s)

Maximum range:

airspeed: 39 m/s (34% faster than max. endurance)
endurance: 5.85 h, fuel flow: 12.8 kg/h
range: 821 km, fuel flow: 9.1 kg/100 km
"fuel flow per airspeed": 0.233 (kg m)/(100 km s)

Carson's speed:

airspeed: 51 m/s (31% faster than max. range)
endurance: 3.86 h, fuel flow: 19.4 kg/h (52% more than max. range!)
range: 709 km, fuel flow: 10.6 kg/100 km (16% more than max. range!)
"fuel flow per airspeed": 0.208 (kg m)/(100 km s) (11% less than max. range!)

It's the same as if you're driving a car on the highway. Let's take one that e.g. uses 5 L / 100 km when driving 80 km/h. Your range with a 50 L tank would be 1000 km, but let's say you only want to go 500 km, which would take more than 6 hours. If you now choose to go 130 km/h, fuel consumption will go up to e.g. 6 L / 100 km, enabling you to cover the 500 km in a bit less than 4 hours, but the fuel consumption would go up - both per distance, as given above, and per time (6 L / 100 km at 130 km/h is 7,8 L/h, whereas 5 L / 100 km at 80 km/h is only 4 L/h - this shows the large difference in power required).
Your speed increases more than your fuel consumption per distance, that's why the higher speed can be considered "better" if you value your time more than your money. Typically want to cover a certain distance by getting from one place to the other, not by simply driving for around for a given amount of hours.

Sorry still don’t understand. A site I was reading states that  “Hence, at Carson speed we may consider the speed for which the ratio fuel flow/knot of airspeed reaches a minimum”.  Doesn’t this mean a small  increase  in fuel consumption for a relatively greater increase in speed. Doesn’t that equate to a better fuel consumption and therefore a better range.

@Ronald  An extreme example: up to Carson Speed, each additional knot costs you an additional 1 oz of fuel per hour and reduces your range slightly. Above Carson Speed, each additional knot costs you 1 gallon/hour and reduces your range drastically (like an afterburner - it's an after-prop!).  The real world has curves, not dramatic inflection points like this, but that's the idea. The fact that additional airspeed becomes more costly above Carson Speed doesn't mean that it isn't somewhat costly (to FF & to range both) above the lower max range speed.

@Ronald No, the best range is at best L/D. The idea is more "I want to fly a given distance from A to B, and my time is worth money.", so Carson is willing to spend more money on fuel while getting from A to B faster. Flying at Carson's speed as opposed to best L/D will decrease travel time by 25%, but only increase fuel consumption by 15%. And right around Carson's speed is where flying faster gives you the best "return on invest", meaning the increase in speed per additional fuel flow is greatest.

That site is wrong. The fuel flow (mass per time, proportional to power) will increase by more than 50%, but the total fuel used for flying a given distance will only increase by 16%, because you take ~30% less time to cover that distance. –
Raketenolli

Ok. This is my question. If total fuel used for a given distance will only increase by 16% for a 30 knot increase in airspeed why don’t I get a greater range at Carson speed than at best range speed. –
Ronald

@Ronald A higher total fuel flow per unit of distance will never result in a better range. You just get to your destination in less time (and drain the tanks in less time).

I am not getting this.  Let’s take this to extremes. If I increase fuel flow by 1% and I get a 100 knot increase in airspeed. That’s going to improve fuel consumption and increase range.

I'm guessing you're still thinking of fuel flow as "fuel per time", which is true, but also the fuel consumption per distance increases, like Sanchises said above. So your range decreases, but you get to your max. range a lot faster. I've added some numbers and further explanation to my post.

Yeah. Makes sense to me now. Thanks guys. Appreciate the help.

Could you have a look at these numbers again please. Your  saying at Carson speed the "fuel flow per airspeed": (0.208 (kg m)/(100 km s) is less than the fuel flow per airspeed (0.233 (kg m)/(100 km s) at maximum Range. Thank you.

Yes? You take the fuel flow in kg/(100 km) given in the "range" line in my list, and divide it by the airspeed, e.g. for maximum range: (9.1 kg/(100 km)) / (39 m/s) = 0.233 (kg m) / (100 km s). I don't see anything wrong with the numbers? The units look weird, but that's how it is when you have a concept like "fuel flow per airspeed".

There are three airspeeds that are related:

Ve –> Speed for maximum endurance -> maximum flight time per unit of fuel

Vr –> Speed for maximum range –> maximum distance per unit of fuel

Vc –> Carson's speed –> minimum fuel consumption per unit of airspeed

It can be demonstrated that the three airspeeds are linked by the factor $\sqrt[4]{3} \approx 1.316$
Thus:
$$ Vc \approx 1.316 \cdot Vr $$
$$ Vr \approx 1.316 \cdot Ve $$
Carson's original paper is 'Fuel efficiency of small aircraft'   https://arc.aiaa.org/doi/abs/10.2514/3.57417?journalCode=ja

Why would this speed now called "Carson speed" when for decades before it was known as optimum transport performance? The original at least says explicitly what it is.


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When should cross country flights occur in flight training
Question about 141 flight schools.  I'm 52 hours into flight training and my 141 flight school wants me to do my cross country work prior to a maneuvering stage check, landing stage check and solo endorsement.
Is this a red flag, meaning the school is milking me for my money?
My understanding is there are two 50 NM flights and 150 NM flight that occurs with instructor and then need to be completed individually.
Monetarily  I want to know that I'm endorsed to fly solo before spending the money on the dual cross country.
My school is unwilling to endorse Solo flight until I have fulfilled cross wind landings with winds at 15 knots.
Thank you

Red flag about the school? Or about your current training level?

Please be more specific. I want to edit the title so that it looks like a question but I find hard to guess what you are asking for (is it normal to make cross country flight with 50 flight hours? at what time should first cross country occur? what kind of flight test a flight school is able perform prior to endorsement?) As is, your question seems really broad and unspecific.

There is no prescribed threshold to cross that indicates when XC training should start.
I generally wait until after a student's first solo to start talking about cross country work.  The first solo is a massive psychological barrier and once the student realizes that they really can fly a plane by themselves, it gives them confidence to learn new stuff.
That said, you are at a 141 school.  They need to follow their approved training schedule.

Thanks for the input.  They do not seem to be following the approved training schedule.  I'm going to follow up with the FAA.

@ubah NO.  DON'T DO THAT YET.  Talk to your instructor's boss, then that guy's boss.  DO NOT jump straight to the FAA.  They are not there to help you and you may end up worse off when you have to start over with a new school.

Yes, going to the FAA will red flag you as a student!

Ok.  I'll bail on that.  I already spoke to the chief flight instructor who is married to the owner.  Have an app the records all calls.  It was very professional, maintained a positive attitude throughout.  The sales pitch was just awful.  Basic message was stick to the program, fly my cross country without the manuever and landing stage checks.  And without the solo endorsement, which will be based on cross wind landings with 15 knot winds.....meet with a different school tomorrow.

It seems that you already have a very contentious relationship with the school.  Good luck.  FYI, recording a phone conversation without the consent of the other party is illegal in many states.

Its only illegal if i publish it.  If the judge doesn't know it happened then its not a crime.

I'm sure your next school will be lucky to have you as a student.

We can close this.  I got the answer I was looking for.  Need to get the school to follow the approved training program.  Since they have deviated I plan on leaving.  Thank you for the input.


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Does a standalone EASA Multi-Engine Instrument Rating exist?
I'm starting an A320 type rating in a couple of months, but my MEP/IR is expiring before that. The pre-entry requirements were only a ME IR. Got me wondering: can I just revalidate my ME IR and let my MEP expire? In other words: is there such a thing as a standalone ME IR (meaning no class or type rating that goes with it)? Thanks and happy landings!

I think you need to modify your question to include jusitriction (FAA, EASA, CASA, JAA, etc) Without this information, your question is unanswerable.

Thank you for your reply. I updated my question.

What is a TR? What country are you talking about?

@Terry TR is Type Rating and country shouldn't matter. As long it's under EASA regulations it should be valid in all its member states (which is where I'll be operating).

If your question is  "do standalone ME IR exist?" you should write it as is in the title. The title helps navigate through questions and have an idea of what is asked about the subject. Many question may exists speaking of multiengine instrument rating,thus the title should be more precise.

@ManuH thanks for the suggestion

MEP? Multi engine propellor? There's no difference between multi-engine ratings depending on engine type.

@jwenting that's not at all what I'm stating or asking.

it's exactly what you're asking as written.

It's possible to revalidate ME/IR every second year in the sim.

This was just published in CAP1855 from the CAA a few days ago, which may answer your question:

Instrument Rating validity, revalidation and renewal for aeroplanes and helicopters
To revalidate, you must hold a valid relevant class or type rating, unless the IR revalidation is combined with the renewal of the relevant class or type rating; no “empty” IR to be endorsed
The UK CAA has deferred this requirement until 10 January 2020 as stated in ORS4 N.1329.
References: Subpart G, FCL.625, FCL.625(A), FCL.625(H))

Exactly what I'm looking for. Thanks a lot!

I'll answer this from a USA perspective.
If you hold a multi-engine rating and/or an instrument rating, they DO NOT expire. Currency determines if they are useable.
If you are getting a new rating (type rating) when that is issued it revalidates the ME and the Instrument rating. You are then current with both and the new type rating. The ME and Instrument ratings are NOT airplane specific, unless a type rating is required for the airplane.
Roger G, ATP (with many type ratings) CFII A&P IA AD.


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What are some common aileron failures?
There are a few parts to the control system for an aileron in a light aircraft, such as the rods, bellcrank, cables, chains etc. What are the main reasons for aileron failures and what parts wear the most during their time?

The words "common" and "failure" do not belong in the same sentence when it comes to ailerons.

Probably hangar rash, but that's just a guess.

Hangar rash isn't a failure mode.

On older (or perhaps all) 177's, the vent tube for the fuel tanks is run between the aileron and the wingtip, the tube might come dislodged and jam the aileron. I check the tube and it's clips every time I fly.

You tagged this [tag:cessna-172], are you looking specifically for 172 answers? If so, you may want to call that out in the question body itself, just to make it more clear.

A tongue-in-cheek answer might reference some sort of failure in the brain of the mammal whose hand is attached to the yoke or stick attached to the linkages attached to the aileron.

When talking about that sort of thing, you're really looking for single-point-of-failure weak links that are difficult to detect during a walkaround or inspection.  Hinges don't normally just let go with no warning.  They get loose and sloppy first.  Same with bellcrank bearings, rod ends, and pulley bearings.  Things in general on airplanes can get really loose and sloppy and still work because there are large fudge factor margins built into the design (like, a crack on a piano hinge might have to progress to 80% across it before normal loads would make it break).
If you're talking about "aileron failure" as in "spontaneous complete loss of aileron control", this is an extremely rare event.  If you had to pick something, you'd generally look for items that can just degrade with no obvious signs and then suddenly let go.  On the typical single-loop cable run control system you'll find in a Cessna or similar, that weak link is the control cable itself, at swages and where the cable bends to go around pulleys, or from corrosion.  The check for cable degradation, besides visual inspection, is to wrap a rag around the cable and pull it along feeling for wire snags (or you can use your bare hands if you enjoy the potential for surprise searing pain).
For cable corrosion, it can seem attractive to go with stainless cable, but stainless cable is weaker and SS doesn't like to rub against itself and the cable tends to fret and break down at pulley bends (why aircraft SS cable has a lubricant in it - the identical-looking SS cable you get at the hardware store doesn't).  In the transport world and in GA I think, you will find mostly galvanized carbon steel cable, which is stronger and doesn't break down as readily (the zinc is a very effective inter-strand lubricant, as well as corrosion preventative).
In terms of overall wear and tear, generally it's parts with moving bearing surfaces that see concentrated loads.  The aileron hinge closest to the control rod will always get loose and sloppy first.  The bearings in the bellcrank in front of the aileron where the bearings are under a constant radial pre-load from cable tension, on top of flight loads.  Pulley bearings at 90 degree bends where radial loads are high.  Pulleys and bearings oriented horizontally go bad sooner than vertical ones because the oil component of the grease in the bearing leaks out over time and the bearing rots out.
In the end though, most of the degradation isn't wear and tear, it's degradation from lack of use.  Like cars, airplanes do not do well sitting stationary for long periods, especially outside.

Flutter due to excess airspeed is one of the more common failure modes for ailerons that operate via a mechanical linkage.   In some cases even adding an extra coat of paint or gel coat to the ailerons without careful rebalancing can promote flutter at lower-than-expected airspeeds.  (See service bulletin or Airworthiness Directive on Libelle sailplanes.)
This answer assumes that problems that develop gradually over time and can be detected by  normal preflight inspections plus regular annual inspections are outside the intended scope of the question.  There are undoubtedly many other reasons besides flutter why ailerons and their associated linkages sometimes need maintenance.

I heard of one a long time ago - I think in Undergraduate Pilot Training. Seems an F-86 was undergoing IRAN, (Inspect and Repair as Necessary), sometime in the 50s, and the mechanic working on the aircraft was an older guy with many years of experience working on other types, but new to the F-86. The F-86 ailerons apparently had a space problem in the bottom of the hinge area that required the bolt that attached the control surface to the hinge to be inserted from the bottom, in order to prevent the lock nut that was on the other end of the bolt from binding. This mechanic, in his infinite wisdom, decided that inserting a critical bolt from the bottom, where it could just fall out if the bolt ever came loose, was a bad idea, so he reversed it and inserted it from the top, and put the lock nut on the bottom.
The next time the aircraft flew, when the pilot attempted to roll the aircraft to turn out of traffic, the aileron jammed and the aircraft (and pilot) were both lost.
As I was told the story, apparently, the accident investigation officials, after determining the cause, elected to not inform the mechanic of what he had done, but simply had him reassigned to an area where he could not repeat his mistake. Don't know if this is true...


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Who checks the instruments and avionics before the crew boards an aircraft?
Before the crew boards the aircraft for a commercial airline flight, is there anyone who checks the instruments and avionics system? I'm sure the engines are checked  but what about the avionics systems, instruments, sensors,etc.?
There was an accident due to an incorrect reading of the pitot sensor, is there somebody to check it?

Maintenance operations have specific checks by the maintenance team. Maintenance operations, and inoperative systems are tracked in a log which is read by the crew when they get the aircraft. There is no reason to expect sudden system failures between flights. When no maintenance is required, the crew performs normal procedures to activate systems. The captain performs a visual check of the aircraft, the pre-flight walk-around. The copilot / first officer is usually in charge of setting the cockpit before the first flight of the day. Readings are cross-checked for compliance.

If you can provide more information about the accident you mention, so that the flight can be identified, then we may provide additional details about possible errors in detecting malfunction, based on the safety board report.

Why specifically "before crew board the aircraft"?

In the accident ,captain's instrument and first officer's instruments showed different reading,this was due to one of the pico sensor was blocked with mud before flight, both of crew where confused,even autopilot system couldn't work because of two entirely different reading,i don't remember the aircraft information

Both of the crew did not analyze the instruments ,actually they didn't compare the instruments,RIP both of them

What is a pico sensor?

Are you perhaps referring to AF 447? That was a failure of the pitot system, not due to mud but ice.

In general, nobody else does.
When pilots arrive to an aircraft for the first flight of the day, extra checks are performed by the crew to verify aircraft systems are all working properly.  These checks are done once per day and subsequent flights will perform abbreviated checks instead.  A visual walk around inspection is accomplished before and after every flight.
Maintenance will check on airplanes at required intervals but outside of specific checks (associated with the carriers approved maintenance program) the only time they'll check something like a pitot tube is if the crew notes a problem and writes it up.  At that point maintenance will address the issue to return the aircraft to airworthiness.

Before crew board the aircraft for a commercial airline, is there any person who checks the instruments and avionics systems

In between flights there is no check of Pitot tube operation from the cockpit prior to crew arrival.

There was an accident due to incorrect reading of pico sensor

There have been a few accidents where one pitot tube (not pico sensor) was blocked (e.g. by the actions of a mud dauber wasp). This can lead to inconsistent readings in the two pilot's displays and/or with data fed to an autopilot.
One example was Birgenair 301

In theory the pilot in command (the Captain) has responsibility for making sure everything works before flight. On small aircraft responsible pilots always check all instruments and sensors. For example, a pitot tube will be checked to make sure that it is clear and that no spider or mud wasp has taken up residence therein. Another key check is to make sure the antennas are all there and firmly attached. Most checklists have at bare minimum an "instruments" line item and may have many line items for specific instruments. When a pilot does the "walk around" they will specifically examine each and every external sensor such as static ports, etc.
Unfortunately, commercial pilots rarely have time to do such thorough checks, which can be very time consuming for a large aircraft. In many cases they will often not even do a walk around, even if their airline's policy mandates it.

your last paragraph is a bold claim with no factual basis. citation needed.

@casey No citation is needed. Next time you take a flight, just watch as the Captain arrives and watch the tarmac. The Captain is supposed to go out the little door and down the stairs and do a walkaround and if he does you will see him. I have been watching flight crews arrive off and on for years and never once have I seen them do a walk around. They just go straight to the flight deck and stay there.

I flew last weekend on 5 different flights and witnessed proper walkarounds. Also, the FO performs walkarounds not the captain. More often than not this is accomplished when the new crew receives the plane, so that the preflight walk around can also accomplish the post flight walk around. I spent 5 years flying for an airline so if we're going to play the game of anecdotes you are far outmatched.


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Is my company EASA 147 approved? How to get an EASA licence?
I am a bit stuck with my career and just need some advice.
I am currently a technician at Trans Maldivian Airways (Twin Otter). I would like to work in an airline in the future. Is this organization EASA 147 approved?
If so, what are the necessary steps I should take to get the license? If not what should I do? I did not do any basic training and I am completing modules conducted by SR Techniques.

If you remove the what should I do portion of the question, there would be no reason to close as opinion based - the rest of the questions seem rather fact-based. Not sure that job advice is appropriate to SE, but opinion based wouldn't be a reason to close.

I am not sure what you should do, but this company is not EASA 147 certified. Source

Without more detail this would be better as a comment.

@fooot The question was "is this company EASA 147 approved" and the answer is "No".  How does this not answer the question?

@Lnafziger I didn't say that it doesn't answer the question. I see the comment requesting the removal of the "what should I do" part, so this answer does cover it. But it does not provide much detail or explanation.


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What is the function of the tail section on a fixed-wing aircraft?
In a generic fixed-wing aircraft like a Cessna, what is the function of the tail section, i.e. the rudder and elevators?

Stability, and control of all angles (yaw, pitch, roll). Agree you will get a more competent answer on aviation.SE

@JohnRennie Just because it can be on topic somewhere else, doesn't mean it is off topic here. And it also doesn't mean it should be closed. I'm not sure it's a good fit here in either case, but I don't agree with that close reason.

I think as worded this is off topic here because the role of specific airplane parts is not really a question about physics. Sure, it comes down to physics in the end, but in its current form the connection to physics is not clear. A slightly modified version might be on topic, though.

Cut the OP a little slack. We were all newbies once. BTW, I could answer this, but this site does a much better job of it.

Here's another great resource, the King Flight School videos. They are at a pace that's very easy to follow. Here's one that may help answer your question, it is on weight and balance: https://www.youtube.com/watch?v=P_0R0eWOKns

First off lets be clear that a tail section does the same thing on small aircraft that it does on large aircraft.
Second off lets be clear that a tail section is not necessary for flight and there are planes like the Northrop Grumman B2 that dont have a tail section they are affectionally called "Flying Wings".
The short answer is that the rudder (vertical part) controls yaw and the elevator (horizontal part) controls pitch. A plane is capable of movement and similarly adjustment in all 3 dimensional axes. But lets dig in a bit:
The Elevator: This is usually located in the back but by no means has to be. Burt Rutan has implemented many designs that have forward mounted elevators. As to functionality they essentially allow you to alter the pitch of the plane which in turn alters the angle of attack of the wing which furthermore has a direct effect on the lift generated by the wing. In a simple world we could say that when you pitch up you climb and when you pitch down you descend. The reality is that the engines thrust plays a large factor in this. For example if you are flying at a level attitude and increase thrust your will increase speed (and the speed of air over the wing) which will generate greater lift and cause the plane to elevate with out a pitch increase (you will have to apply some force to keep the plane level though). The elevator provides a way to easily alter the pitch of the plane (and angle of attack) so you can climb when you want. Generally speaking you will also add power for the climb. For a more lengthy explanation check out the wikipeidea article.
The Rudder: The rudder is mainly to control the left-right yaw of the aircraft. In a single engine prop plane this can be used to help overcome the left turning tendencies created by uneven force of a spinning prop (P-Factor). In a more general case the rudder is often used to correct yaw when a heavy crosswind is present on landing. It is also used in flight to ensure your turns neither skid or slip (stay coordinated) remember "Step On The Ball". For more reading check out Wikipedia
This picture shows the axes of movement pretty well.

forward mounted elevators you mean canards?

There is more to the empennage than the rudder and elevator. There are the H and V stabs as well

The function of the tail section of a conventional fixed wing aircraft is to provide stability. It also holds two important control surfaces; the rudder and the elevator.
The vertical tailplane, also called vertical stabilizer helps the aircraft's nose facing into the direction of the relative air flow. If the aircraft is in a side-slip, which means the nose of the aircraft is pointing left or right of the direction of the relative airflow, the vertical tailplane will create an aerodynamic moment forcing the nose into the wind.

The vertical tailplane also contains the rudder. The rudder is a control surface which controls the aircraft's yaw moment. The pilot controls the rudder by using the rudder pedals. This allows the pilot to correct disturbing pitch moments which might come from the propeller or to bring the aircraft intentionally into a sideslip, for example for a crosswind landing. During turns the rudder is used to ensure the turn is coordinated (keep the sum of the weight and centrifugal force pointing down the vertical axis of the aircraft)
The horizontal tailplane, or horizontal stabilizer is for longitudinal stability. Usually it provides the horizontal stabilizer gives and aerodynamics down force, thereby keeping the nose up. That may sound counter intuitive, but is essential for the stability of conventional aircraft. When the airspeed increases the aerodynamic forces increase as well. Due to the increase of the lift force on the main wing combined with an increase in down force on the horizontal stabilizer the nose of the aircraft will be forced upward. This causes the aircraft then to trade kinetic energy for potential energy and the speed will reduce again.
The horizontal stabilizer also contains the elevator. This control surface allows the pilot to control the pitch moment of the aircraft. Aircraft can be trimmed such that they maintain a constant pitch angle at constant speed without the need to constantly give elevator inputs. This trimming is done by either a small trim tab on the elevator, but in other aircraft the angle of whole horizontal stabilizer can be changed to trim the aircraft.

Good answers, but let me make it short and simple.
The tail (empenage) mainly contains the vertical stabilizer and the horizontal stabilizer.
This video shows what happens when the vertical stabilizer is lost.
The plane has no way to stay pointed into the wind - very bad.
When the horizontal stabilizer is lost, the plane instantly noses down.
(I tried to find the video of the dam-busters practice where the Lancaster bomber is going along above the water.
It drops the cylinder, which hits the water, bounces up, and slices off the tail of the bomber, which noses into the water, at 100+ mph.)


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Why is there a compressor in an air cycle machine?
I'm a curious high school student. Why is there a compressor after the first heat exchanger in an air cycle machine? I've read that compressing the cold air is for the cause of efficiency because compressing hot air uses much more work than cold air, and the effect of this is that the compressed air temperature increases. But if so, why compress at all when the air itself is going to be cooled again in the second heat exchanger?  Why not leave the compressor and place the turbine as it is from the ACM?

If I understand your question correctly, you don't really understand how the ACM works.
The idea of an ACM is to create 'cool' air. Especially on the ground (imagine landing on your tropical holiday destination), this means that the air must sometimes be made cooler than the outside (ambient) temperature. The second law of thermodynamics (and in this case, the law of common sense) states that you can't just cool your air against the warm ambient air, so you will need to do some special tricks.
In this case, the special trick is compression. You compress the air, which will make it quite hot (increase the temperature) (search for adiabatic compression on the internet). Then, this hot air can of course easily be cooled against ambient air, which happens in the second heat exchanger. In fact, the hotter the air after compression is, the more heat is exchanged in the second heat exchanger. This means the heat exchanger can be made smaller, which is especially useful since air-to-air heat exchangers are inefficient and thus relatively big.
Now, you expand your air again, which will cool it (decrease the temperature). However, since you already cooled it down in the second heat exchanger, it will now be cooler than when it originally encountered the compressor! This makes sense, since you removed energy from the air in the second heat exchanger. So, this way, you can cool the air down further than would have been possible by a heat exchanger (search for reverse Brayton cycle on the internet).
So, this system has two advantages:

It is possible to cool down air beyond ambient temperature, so it functions as 'airco' on the ground or at low altitude.
It is possible to cool hot compressed bleed air with a relatively small, light heat exchanger for pressurizing your cabin while not cooking your passengers medium-rare.

Very nice answer

Because there is an expansion stage, the energy of the compressed air can be used to run a second compressor. Doing this allows to cool the precompressed air before it is fed to the second compressor, thus improving overall efficiency.
For the same reason, some turbocharged engines use two turbocharging stages with an intercooler between the compressor stages. It keeps the volume of air low and allows for smaller, more efficient turbochargers.
You could also extract the air at a later stage of the engine compressor, cool it and expand it in a diffusor, but then the engine compressor has to do more work and the cooler needs to be bigger due to the higher temperature and lower density of the compressed air. But somehow you still need to shed the pressure energy in the air, and running it through a turbine is an ideal way to achieve this.

You're right on the money about compressing hot air being less efficient. That's exactly why the air cycle machine (ACM) has that two-stage approach with the first heat exchanger.
Here's the breakdown:

Pre-Cooling for Efficiency:  The first heat exchanger cools down the bleed air from the engine before it enters the compressor.This is because compressing cooler air takes less energy compared to hot air. It's like squeezing a softer balloon versus a fully inflated one.
Pressure Makes Cold (Sort Of):  Even though the pre-cooled air gets compressed, raising its temperature, the overall cooling effect comes from the next stage. The compressed air then travels to the second heat exchanger where it loses heat to the surrounding environment (or another cooling source).
The Turbine's Role:  The compressed air then expands through a turbine, which extracts energy to drive the compressor. This expansion cools the air significantly, achieving the actual cooling effect you mentioned.

The ACM has a nozzle in the turbine which compression and rapid expansion of air through the conical holes of the nozzle, and inducer and exducer blades of the turbine wheel pulling and pushing air at high RPM's produces the cold air.  Primary cools down bleed air from engine for perameter required,  then temperature leaving compressor discharge into secondary heat exchanger cooling down air sufficiently for turbine inlet air temperature requirements and turbine discharge temperature.  Example,  testing HONEYWELL ACM for 757, and 767, from primary to compressor inlet is 197 to 207 deg. F.  comp discharge temp is about 285 deg. entering secondary then exiting at 205 deg. F entering turbine inlet and outlet discharging at 25 deg. fer. or less. from turbine scroll.  Also, the RPM is 46K to 51K.  Some smaller ATR ACM's go over 100k.